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authorPeter Hurley <peter@hurleysoftware.com>2014-05-03 14:04:59 +0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-05-03 18:13:05 -0400
commit4291086b1f081b869c6d79e5b7441633dc3ace00 (patch)
treec37b5ba253d23f6ece2ce8b95b94a909792666f4
parentb790f210fe8423eff881b2a8a93ba5dbc45534d0 (diff)
n_tty: Fix n_tty_write crash when echoing in raw mode
The tty atomic_write_lock does not provide an exclusion guarantee for the tty driver if the termios settings are LECHO & !OPOST. And since it is unexpected and not allowed to call TTY buffer helpers like tty_insert_flip_string concurrently, this may lead to crashes when concurrect writers call pty_write. In that case the following two writers: * the ECHOing from a workqueue and * pty_write from the process race and can overflow the corresponding TTY buffer like follows. If we look into tty_insert_flip_string_fixed_flag, there is: int space = __tty_buffer_request_room(port, goal, flags); struct tty_buffer *tb = port->buf.tail; ... memcpy(char_buf_ptr(tb, tb->used), chars, space); ... tb->used += space; so the race of the two can result in something like this: A B __tty_buffer_request_room __tty_buffer_request_room memcpy(buf(tb->used), ...) tb->used += space; memcpy(buf(tb->used), ...) ->BOOM B's memcpy is past the tty_buffer due to the previous A's tb->used increment. Since the N_TTY line discipline input processing can output concurrently with a tty write, obtain the N_TTY ldisc output_lock to serialize echo output with normal tty writes. This ensures the tty buffer helper tty_insert_flip_string is not called concurrently and everything is fine. Note that this is nicely reproducible by an ordinary user using forkpty and some setup around that (raw termios + ECHO). And it is present in kernels at least after commit d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty layer to use the normal buffering logic) in 2.6.31-rc3. js: add more info to the commit log js: switch to bool js: lock unconditionally js: lock only the tty->ops->write call References: CVE-2014-0196 Reported-and-tested-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Jiri Slaby <jslaby@suse.cz> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Alan Cox <alan@lxorguk.ukuu.org.uk> Cc: <stable@vger.kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--drivers/tty/n_tty.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 41fe8a0..fe9d129 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2353,8 +2353,12 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
if (tty->ops->flush_chars)
tty->ops->flush_chars(tty);
} else {
+ struct n_tty_data *ldata = tty->disc_data;
+
while (nr > 0) {
+ mutex_lock(&ldata->output_lock);
c = tty->ops->write(tty, b, nr);
+ mutex_unlock(&ldata->output_lock);
if (c < 0) {
retval = c;
goto break_out;